As a method of sensing a photograph with appropriate lightness, a method of measuring the average brightness of a scene to be sensed, and controlling the shutter speed, aperture value, and the like of a camera is known. Also, an exposure control method based on so-called evaluation metering for obtaining an appropriate exposure value by segmenting a scene into predetermined regions, and calculating the average brightness value by weighting brightness values measured for respective regions is known.
However, in an image which is sensed in a so-called backlight scene in which an object is especially dark compared to the background, the object inevitably becomes dark. In order to sense a photograph with appropriate lightness in such backlight scene, the exposure value of the camera must be set to sense a relatively lighter image (exposure correction). However, exposure correction requires not only troublesome operations but also skills. Even when an appropriate exposure value can be set for the object, the background often becomes too light.
As an analog photograph technique for obtaining an image with appropriate lightness in a backlight scene in which it is difficult to appropriately decide the lightness of an image, so-called dodging which is done in a dark room is known. By applying dodging, a photograph with appropriate lightness can be obtained.
As a method of implementing dodging in digital image processing, for example, a report: Jobson et al., “A Multiscale Retinex For Bridging the Gap Between Color Images and the Human observation of Scenes”, IEEE TRANSACTIONS IMAGE PROCESSING, VOL. 6, NO. 7, July 1997, is known (reference 1). In this method, differential processing between a component obtained by log-converting a digital image and a low-frequency component of the log-converted component is executed. Then, the image is changed for the better by processing a light component in the low-frequency range of the digital image to be darker, and a dark component in the low-frequency range to be lighter.
Also, a report; Reinhard et al., “Photographic Tone Reproduction for Digital Images”, acm Transactions on Graphics, July 2002, Vol. 21, No. 3, is known. This report also proposes a method of obtaining a dodging-like effect in digital image processing using the brightness component of a digital image and its low-frequency component.
Of course, exposure correction is not limited to still images, and is similarly required in moving image sensing. Since a moving image can be considered as a set of a series of still images which are continuous along the time axis, the aforementioned image correction can be easily applied to the moving image.
However, playback processing of digital moving image data has high processing cost and requires high processing performance for a playback apparatus. In addition, in order to implement image processing that can obtain a dodging-like effect, extraction processing of the low-frequency component or the like must be added to the playback processing of a moving image, thus requiring higher processing cost and higher processing performance. Of course, these requirements lead to a high price of the apparatus.
As a known technique associated with such image processing, an apparatus described in Japanese Patent Laid-Open No. 2000-149014 is known.
In recent years, it is possible to play back digital moving image data using application software which runs on a personal computer (PC). However, a Central Processing Unit (CPU) or Graphical Processing Unit (GPU) are required to have high processing performance. If the processing performance is insufficient, the processing cannot be done in time, and some frames are not played back upon playback of a moving image, thus causing so-called drop frame.
In terms of a reduction of the processing load of digital moving image data, a technique described in Japanese Patent Laid-Open No. 2002-77723 is known. This technique detects a scene change based on difference images upon coding, decides a correction method using a predicted image upon the scene change, and applies the decided correction method until the next scene change. However, this technique cannot be applied to image processing that obtains a dodging-like effect.